Depends on the cam. A small cam will change your power curve and depending on the cam it will still run fine, but over do it and you've either wasted money on a cam you can only sell as used or be shoppin' for heads. I've got an E303 cam I was told will work fine with a stock 5.0, but also works on some s/c applications, but I haven't installed it yet to confirm that.
But the answer to your question is yes, you can put an aftermarket cam in your 5.0, just do your research on which one. But unless your current cam is f'ed up I wouldn't change it by itself.
That is a very vague question. I would highly recommend doing alot of research on the topic before spending any money or doing any work. Thats like asking if a 460 can be swapped into a fox.
my friend put an F-cam in a stock motor and even with a dyno tune its only putting 170 to the wheels. sounds awesome until I show him the tail light on my ruckus scooter.
Many things to do before a cam install. Back in highschool 1995-96, I spent $1000+ on E cam, GT40 upper/lower, 65TB, k&n filter. The car sounded meaner, but it did not run like I was expecting. It was a waste of money cause I had stock heads, stock headers w/flowmasters, and stock everything else.